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A2 Daily A Level Physics question

2026-05-07 OCR A Electric fields (Module 6.2) Module 6.2 Electric fields — Uniform fields; E = V/d; motion of charges between parallel plates (OCR A, A Level)

An oscilloscope uses horizontal parallel plates to deflect an electron beam that travels midway between them with constant horizontal speed. The plate separation is doubled while, at the same time, the potential difference across the plates is doubled. The plate length, beam speed and screen distance are unchanged. What happens to the beam’s vertical deflection at the screen, and why?

  1. A It increases, because a larger potential difference creates a stronger downward force.
  2. B It decreases, because the wider gap weakens the field between the plates.
  3. C It becomes zero, because the effects of increasing potential difference and spacing cancel completely.
  4. D It stays the same, because the field strength between the plates is unchanged when both changes are made. (correct)

Answer

The correct answer is D.

Correct: D — It stays the same, because the field strength between the plates is unchanged when both changes are made. A ignores that doubling the separation at the same time keeps the field the same when the potential difference is also doubled. B ignores that the potential difference is also doubled, restoring the original field strength. C is wrong because the field does not become zero; doubling both quantities keeps the field constant, not cancels it. D is correct since the field (and hence the acceleration and exit angle) is unchanged, so the deflection at the screen is unchanged.

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